The present invention is related to a linear hydraulic motor and piston assembly, and more specifically, the present invention relates to securing the piston head to the piston rod in a linear hydraulic actuator.
Linear hydraulic motors are utilized in a variety of applications in machines and work vehicles to accomplish power operation and/or remote control of parts and components. For example, hydraulic motors are used as linear actuators for work implements and tools in agricultural and construction equipment. The operation or remote control is accomplished by an operator controlling the supply of pressurized hydraulic fluid to the motor from a control at the operator""s station.
These hydraulic motors may be subject to frequent operation and may operate at pressures above 3600 psi pounds per square inch or more. In some work situations the piston assembly can be subject to large dynamic forces causing yielding or damage. For example, large forces are generated when a moving part reaches the end of stroke or an implement is slammed into a hard object, such as when a shovel or bucket of construction equipment is dropped onto hard pavement.
In some hydraulic motors, a conventional method of attaching a piston rod to a piston head is by means of a pivoting piston pin. The pivoting pin allows the piston to maintain alignment in the cylinder bore while permitting some angular displacement of the piston rod. However, in hydraulic cylinders that are used on agricultural or construction equipment and the like, it is conventional to pivotally mount the hydraulic cylinder to the frame with the end of the piston rod being pivotally mounted to a reactive member or tool. Thus, it is not necessary for the piston head to be pivotally mounted to the piston rod. Therefore, several types of non-pivoting attaching means have been developed for connecting the piston head to the piston rod.
One common construction utilized for securing the piston head to the piston rod end in linear hydraulic motors is a threaded or screw type connection. As shown in U.S. Pat. No. 3,885,461, one known screw type connection uses a threaded engagement between external threads on the rod and internal threads in the piston head and on a lock nut that is disposed adjacent to the piston head. The nut bears against the piston head and holds it in the desired position on the rod. As shown in U.S. Pat. Nos. 4,089,253 or 4,917,003, another screw type connection uses internal threads in an opening in or through the center of the piston head. Mating external threads are provided on the rod that extends into or through the piston head.
In some hydraulic cylinders for agricultural and construction equipment, as shown in U.S. Pat. No. 5,026,246, it is known to have a threaded bolt inserted through a through-hole in the piston head and threaded into a threaded bore in one end of the piston rod. The bolt is tightened to a proper clamp load to secure the piston head on the piston rod. A problem with this type of assembly is that considerable clamping torque is necessary to assure the attachment of the piston head to the piston rod. The clamping load of the bolt could possibly subject the metal of the machined piston head to excessive loads. The metal of the piston head could yield or be crushed when the piston head reaches the end of stroke or is abruptly stopped such as by an impact.
Piston heads are usually made of a softer material than the bolts. It is desirable to make the pistons out of a free-machining steel to simplify the machining of the delicate machined grooves needed for seals and bearings. A hardened washer is used to spread the high clamping load over a larger area on the piston to prevent crushing damage. If piston crushing occurs, the result is a loss of pre-load in the bolted connection, lowering the amount of load the piston assembly can withstand. If enough piston crushing occurs the bolt can lose all pre-load, at which point the bolt can fall out of the assembly, causing failure.
Fasteners manufactured with captured washer elements are called SEMS in the industry (see ANSI/ASME B18.13, 1987) and generally have been used to attach assemblies such as automotive interiors. The majority of SEMS fasteners have a diameter of less than xc2xexe2x80x3 for the bolt portion. They generally are not used as a means of producing a high-performance bolted joint, but rather the benefit of the SEMS comes from labor savings (faster assembly times) and error proofing (making sure a washer is always used.) The new use of SEMS fasteners in this invention is to economically obtain a high-strength, high-performance bolted joint with error proofed assembly.
Thus, it is an object of the present invention to provide a simple, reliable and economical mechanism for connecting the piston head and piston rod assembly that reduces the potential for premature failure or stress concentrations on the piston head caused by currently known attaching mechanisms during extreme but not uncommon work situations.
One embodiment of the invention relates to a piston assembly for use in a hydraulic cylinder. The piston assembly includes a piston head configured to slide axially within the hydraulic cylinder in response to changes in hydraulic pressure. The piston head has an axial through-hole. A piston rod has a proximal end axially abutting the piston head and a distal end extending outward through an end of the hydraulic cylinder. The proximal end of the piston rod has a threaded bore co-axial with the axial through-hole in the piston head.
A bolt has a bolt head and a shank. The shank has an unthreaded portion adjacent the bolt head and a threaded portion away from the bolt head. The bolt secures the piston head to the proximal end of the piston rod when the bolt shank is inserted through the axial through-hole in the piston head and is threadingly engaged in the threaded bore in the piston rod.
An annular washer is positioned and held captive on the unthreaded portion of the bolt shank between the bolt head and the piston head. The washer has a flat face abutting the piston head so as to concentrically transfer clamping force from the bolt head to the piston head when the bolt is threaded into engagement in the threaded bore in the piston rod. The piston head is secured between the bolt head and the piston rod.
The invention also includes a method for assembling a piston assembly for use in a hydraulic cylinder. A piston head is provided with an axial through-hole. A piston rod is provided with a threaded bore co-axial with the axial through-hole in the piston head. A threaded bolt is provided with a captive annular washer. The bolt is inserted through the through-hole in the piston head and threadingly engages the threaded bolt in the threaded bore so as to clamp the piston head to the piston rod.